1. Field of the Invention
The invention relates to a multi junction solar cell, comprising a substrate like a Ge or GaAs substrate as well as a solar cell structure comprising several subcells deposited on the substrate, wherein the substrate has peripheral side faces and the solar cell structure has a peripheral circumferential surface which runs spaced apart from the side faces.
2. Description of the Background Art
The invention also refers to a method for producing a multi junction solar cell, in particular a concentrator solar cell, wherein the solar cell structures with several subcells are deposited onto a substrate, such as a Ge or GaAs substrate, wherein an active solar cell area is defined as a solar cell structure by an etching, laser, or sawing process with subsequent overetching under formation of a circumferential surface and wherein the substrate is severed by sawing or lasing under formation of peripheral lateral faces into substrate sections for forming the individual multi junction solar cells.
Multi-junction solar cells are principally used in space travel and in terrestrial concentrator photovoltaic systems, CPV, which are increasingly gaining importance on the market for photovoltaics. Extended trouble-free service life is a pre-requirement for the cost-effectiveness of these systems and to gain further recognition. But on the other hand this will require overcoming a combination made up of widely varying environmental influences, such as moisture, temperature fluctuations, and UV light, which have a negative effect with respect to the reliability of individual components, submodules, and modules.
For the solar cell, as core component of a CPV module, the continuous loading with condensed moisture is critical among other things, because this can result in electrochemical corrosion of the substrate material of the cell and the cell structure and furthermore in failure of the solar cell. Since the front side of the solar cell is normally protected by application of a cover glass or an optical element, such as a lens or light diffuser, particularly the ridges of the substrate and the solar cell structure (mesa) will be affected on which photoactive areas of the solar cell are not protected.
FIG. 1 shows prior art which is practiced by some companies, wherein a solar cell is applied onto a conductive copper foil and is covered with a glass. The gap between the cover glass and the copper foil is provided with EVA (ethylene vinyl acetate) encapsulation of the solar cell in order to isolate the solar cell from the environment.
But this sealing technology is complex, however, and adds significant weight because of the cover glass.
A conventional CPV multi junction solar cell consists of a substrate, such as a Ge substrate or GaAs substrate, onto which the solar cell structure is deposited. In this context, the lowest cell of the multi junction solar cell can form itself directly in the substrate material during the deposition process. During the production of solar cells, the size of the active solar cell area (mesa) is defined by an etching or laser process, or a sawing process with subsequent overetching and an upper metallization and antireflective coating are deposited. Finally, the solar cell wafer is severed into individual cells by sawing or lasing.
The term “mesa” is generally known as an elevated area with a flat surface and a steep slope.
The produced CPV solar cells are normally bonded or soldered individually onto substrates, in Fresnel-based systems, for example, or with large surfaces, in dense array modules, for example. In the case of Fresnel optics modules, the substrates with the solar cells are assembled and interconnected on the back face of a module interior. In order to prevent pressure differences between the module interior and the environment, due to temperature and/or air pressure variations, for example, the modules are designed to be open or with a pressure compensation filter.
The filter can contain a membrane material that is impervious to water vapor, such as Gore-Tex, to largely prevent moisture from penetrating into the interior.
Alternatively, the back face of the module can be sealed with a large-surface transparent polymer layer after the complete assembly and interconnecting the substrate with the cells. Such embodiment according to the prior art is illustrated in FIG. 2.
As additional measure against penetration of the moisture into the module interior, the interior is flushed constantly with dry air or dry nitrogen in some systems.
A solar cell can be found in US-A-2009/0159119, the metallization on the front of which is covered by a moisture barrier layer, which can also extend along the side faces of the solar cell. Polyimide, silicon nitride or silicon oxide can be used as a material for the protective layer.
Integrated thin-film solar cells are the subject matter of US-A-2010/0018574.
A metallization which is suitable for high concentrator solar cells is described in U.S. Pat. No. 5,075,763.